1. Field of the Invention
The present invention relates to a dot arrangement determination method and apparatus suitable for forming pseudo gradation images, a method for creating a threshold matrix, a program for implementing the functions of these methods with a computer, and an image forming apparatus that uses a threshold matrix; and particularly relates to a halftone processing technique suitable for an inkjet recording apparatus or another image forming apparatus.
2. Description of the Related Art
In an inkjet recording apparatus, ink is deposited on recording paper or another such recording medium by ejecting ink droplets from the nozzles of a recording head (also referred to as a print head), and prints or images (hereinafter referred to collectively as “images”) are recorded by these dots of deposited ink, but conditions may arise in which ink cannot be ejected from the nozzles for various reasons. For example, when a particular nozzle in a group of nozzles fails to eject, the dots that should have originally been deposited by the nozzle are missing, which results in unintended flaws in the form of lines (streaking) in the recorded image on the recording medium, and the image quality is reduced.
To deal with such problems in the prior art, a method is proposed to achieve a deposition arrangement (dot arrangement) wherein quality reduction in the recorded image resulting from deposited position misalignments due to nozzle ejection failure or ejection direction errors can be prevented (Japanese Patent Application Publication Nos. 2001-88328 and 2002-16803). Other proposals include a technique for performing a pseudo gradation process that improves graininess, that is suitable for photographic images, and that has excellent gradation reproducibility (Japanese Patent Application Publication No. 2002-16802), as well as a threshold matrix for obtaining a dot arrangement in which image quality reduction in areas of low resolution is prevented (Japanese Patent Application Publication No. 2004-15674).
However, in the techniques proposed in the prior art, the technique of reducing nonuniformities (in the line patterns or concentration) resulting from omitted dots (so-called “dot gaps”) due to nozzle ejection failures or the like is not investigated in terms of visual characteristics, and the optimum dot arrangement based on visual characteristics is not determined.
Also, in Japanese Patent Application Publication Nos. 2002-16803 and 2002-16802, the characteristics of the dot arrangement achieved by image processing are described, but an optimization technique is not used in the method for achieving these characteristics.